Vapor pressure monitoring



Aug. 2, 1966 R, A. BROWN ET AL 3,263,491

VAPOR PRESSURE MONI TORING Filed Nov. 16, 1962 null wat'. lt

United States Patent O 3,263,491 VAPOR PRESSURE MONITORING Rupert A. Brown, New Canaan, Conn., Kevin N. Thompson, Whitestone, N.Y., and Robert W. Somers, Glassboro, NJ., assignors to Mobil Oil Corporation, a corporation of New York Filed Nov. 16, 1962, Ser. No. 238,142 8 Claims. (Cl. 73-53) This invention relates to vapor pressure detecting, and more particularly to the monitoring of the vapor pressure of a stream of material such yas commercial liquiiied propane glas.

It is well known that the value of certain materials may be enhanced over and above their product values when such materials are blended with other materials. For example, in the manufacture of commerci-al propane, controlled amounts of ethane may be added to the propane LPG (liquiiied petroleum gas) to increase the Vapor pressure of the propane, which is normally 186 p.s.i.a. at 100 F. The vapor pressure of the commercial propa-ne may be increased in this manner to the maximum per mitted by ASTM standards, which is 210 p.s.i.g. at 100 F. as defined -by ASTM-Dl267-55, Vapor Pressure of Liqued Petroleum Gases."

One of the problems which arises in connection with the upgrading of the value of material such as ethane in the manner pointed out above is the` requirement for the careful control of the amounts of such material which are added to the blending materials. Heretofore it has been common practice to meet `this control problem and to maintain constan-t the consistituency of the blend by means of laboratory testing of vapor pressure. By means of instruments such as disclosed here the blending product is continuously sampled and vapor pressure readings are taken of the samples. Variations in the sample vapor pressure indicates variations in the blend constituency and modificati-ons in the blend are made accordingly. The present invention provides an improvedmethod and means for providing the monitoring required for blending operations .such `as those described above.

In accordance with 4the present invention, a stream of liquid propane is sampled and a pool is formed with the sample in a measuring chamber. The sampling of the stream of propane is continuous so that new prop-ane from the stream is continuously added to the pool. Propane is also continuously removed from the pool and returned to the main propane stream -so that the pool is part of a continuously flowing sample stream. The measuring chamber is immersed in an oil bath maintained at 100 F. and the propane ows through a heat eX- changer in this bath before being -added to the pool. A standard liquid comprising a blend of propane and ethane is selected to have a vapor pressure of just below 210 p.s.i.g. at 100 F., which vapor pressure is the target value desired in the main stream of commercial propane. The standard liquid is placed in a closed container, which is immersed in the pool of propane forming part of the sample stream of propane. A differential vapor pressure detector, which is also immersed in the oil bath maintained at 100 F., is connected to detect any difference in pressure between the closed container containing the standard liquid and the measuring chamber containing the pool of liquid propane forming part of the sample stream of propane. The fac-t that the standard liquid is in a container immersed in the sam-ple stream of propane ensures that the temperature of the standard liquid is precisely the same as the temperature of the sample stream. The oil bath maintained at 100 F. keeps both the standard liquid `and the pool in the sample stream as close as possible to 100 F. Because 3,263,491 Patented August 2, 1966 of this combination of features, more accurate monitoring of the vapor pressure of the liquid stream is obtained than can be obtained with the systems of the prior art. The fact that the differential vapor pressure detector is also immersed in the oil bath further'increases the accuracy of the monitoring. The measuring chamber is vented with a capillary tube to permit non-condensibles to escape from the measuring chamber. This capillary tube is essential when the liquid being monitored is a mixture.

Accordingly, a principal object of the present invent1on is to provide an improved system for monitoring the v-apor pressure of liquids.

Another object of this invention is to provide a more accurate system for monitoring the vapor pressure of liquids.

A further .object of this invention is to provide a more 1accurate apparatus for monitoring the vapor pressure of 1qu1 s.

A still further object of (this invention is to provide a more accurate method for monitoring the vapor pressure of liquids.

A still further object of .this invention is to provide an improved system for monitoring the vapor pressure of liquid propane.

Further objects and 'advantages of the present invention will become readily apparent as the following detailed description of the invention unfolds and when taken in conjunction with the single ligure of the drawings which schematically illustrates the apparatus and method of the present invention. v i

As shown in the drawings, the commercial propane, the vapor pressure of which is to be monitored, flows through a line 11. A sample of the propane is tapped from the line 11 and llows through a valve 13 and then through a strainer 15. From the strainer 15 the sample of the propane flows through a line 19 which passes into a tank 21 through a seal 20. Within the tank 21 the line carrying the sample of propane is surrounded by `an oil bath which is maintained at a controlled temperature. Within the oil bath in the tank 21, the sample of propane passes from the line 19 through a coil 23, Where the sample is heated to the temperature of the oil bath. After passing through the coil 23, the propane sample ilows to a valve 25 which is .also immersed in the oil bath within the tank 21. The valve 2S admits a controlled amount of the propane sample t-o a measuring chamber 27, which is also immersed in the oil bath within the tank 21. The valve 25 permits just enough of the sample to flow into t-he chamber 27 to maintain the chamber 27 20% yfull of liquid propane. Within the chamber 27, the vapor and liquid propane approach an equilibrium. Liquid is withdrawn from the chamber 27 from the bottom lthrough a line 29, which passes out through the wall of the tank 21 through a seal 32. The propane withdrawn through .the line 29 flows through a ilow rate indicator 33 and then through a tlow regulator 35. From the ilow regulator 35 `the propane flows back to the line 11 at a suitable point. The ilow regulator 35 maintains the rate of propane tlow out of the chamber 27 through the line 29 constant. A vent tube 37 passes through the top of the chamber 27 in a seal 39 and through the top of the tank 21 in a seal 41. A capillary tube 43 is centrally mounted in the tube 37 by means of a seal 45 which surrounds the capillary tube 43 and completely seals olf Ithe space between the capillary tube 43 and the Vent tube 37 so that the top of the chamber 27 is open to the vent tu-be 37 only through the capillary tube 43. The vent tube 37 leads to atm-osphere or to a flare line. The Capillary tube 43 permits a very small `amount of vapor -to be continuously Y released from the top of the chamber 27. This operation serves to purge traces of non-condensibles which otherwise would accumulate within the chamber 27. A screen 47 may be used to cover the bottom of the vent tube 37 to prevent the capillary tube 43 from clogging. A differential pressure measuring transmitter 49 is also mounted within the tank 21 and immersed in the oil bath. A closed container comprising a bulb 51 is immersed in the pool of propane liquid Within the chamber 27. A line 53 transmits the vapor pressure within the bulb 51 to the transmitter 49. The line 53 passes through the wall of the chamber 27 at the top thereof in a seal 55. The pressure transmitter 49 also has an input line 57 which transmits an applied input pressure to the transmitter 49. The transmitter 49 produces an output signal representing the difference in the pressure transmitted to the unit 49 through the line 57 and the pressure inthe bulb S1 transmitted to the unit 49 through the line 53. In the system of the present linvention the line 57; leads into the upper part of the chamber 27 through a seal 59 and senses the vapor pressure in the upper part of the chamber 27. Thus the differential transmitter 49 will produce an output signal representing the difference in the vapor pressure Within the bulb 51 and the vapor pressure within the chamber 27. The bulb 51 is permanently filled with a standard fluid, which is a standard propane and ethane blend, the vapor pressure of which is selected to be at the desired target value of the propane being monitored flowing through the line 11. This target value, as pointed out above, is slightly below 210 p.s.i.g. at 100 F. If the vapor pressure of the sample propane liquid flowing through the chamber 27 from the line 11 is above 0r below this desired target value, it will be indicated by the output signal from the unit 49.

The oil bath within the tank 21 is maintained at a constant temperature, which in the specific embodiment of the present invention is 100 F. by means of a coil of tubing 61. The temperature of the oil within the tank 21 is sensed by means of a bulb 63 which transmits a signal representing the temperature to a control unit 65. In response tothe temperature sensed by the bulb 63 the control unit 65 positions a valve 67 which controls the amount of heating fiuid flowing through the tubing coil 61. When the temperature sensed by the bulb 63 starts to drop below 100 F., the control unit 65, in response thereto, opens the valve 67 a little wider, permitting more heatinguid to ow through the coil 61, thus causing the temperature of the oil within the tank 21 to be raised again back to 100 F. Likewise, when the temperature of the oil bath in the tank 21 starts to rise above 100 F., th-e control unit 65, in response thereto, moves the valve 67 towards its closed position to reduce the amount of heating fluid flowing through the coil 61 to bring the temperature of the oil in the tank 21 back to 100 F. The oil in the tank 21 is continuously circulated by means of a propeller 71 to maintain a uniform temperature throughout the oil bath. The oil bath thus keeps the pool of liquid propane formed in the measuring chamber 27 and the standard liquid in the bulb 51 as close as possible to 100 F., the temperature at which the target value of vapor pressure is measured. Because the bulb 51 is immersed in the pool of liquid propane in the chamber 27, any slight variation in the temperature of the pool will cause the same change in the temperature of the standard `liquid in the bulb 51. Such small variations in temperature of the pool in the measuring chamber 27 will not cause errors in the output signal from the transmitter 49 since the changes in vapor pressure caused by such variation in temperature will occur in both the pool and in the standard liquid. Thus the transmitter 49 will very accurately detect any variation in the vapor pressure of the propane stream in the line 11 from the target value. This accuracy is further augmented because the transmitter 49 itself is immersed in the oil bath.

Alternative arrangements for providing a constant temperature for the system may be employed. For example, the bath may be heated electrically and the liquid propane stream itself may be used to provide a bath which is closely controlled in a known manner to have a relatively constant temperature. These and many other modifications may be made to the above described specific embodiment of the invention without departing from the spirit and scope of the invention, which is defined in the appended claims.

What is claimed is:

1. An apparatus for continuously determining vapor pressure of liquids comprising a measuring chamber, a closed container suitable for containing a standard fluid within said measuring chamber, means to continuously introduce liquid into said chamber, remove such liquid from said chamber, and maintain a pool of such liquid in said chamber immersing said closed container, means to detect any pressure differential between said container and said chamber, and means to provide a constant temperature bath immersing said measuring chamber.

2. An apparatus for continuously detecting vapor pressure of liquids comprising a measuring chamber, a closed container suitable for containing la standard fiuid within said measuring chamber, means to continuously introduce liquid into said chamber, remove such liquid from said chamber, and maintain a pool of such liquid in said chamber immersing said closed container, means to detect any pressure differential between said chamber and said container, and means to maintain the pool of liquid in said chamber at a constant temperature.

3. An apparatus for continuously detecting vapor pressure of liquids comprising a measuring chamber, a closed container suitable for containing a standard fiuid in said measuring chamber, means to continuously introduce liquid into said chamber, remove such liquid from said chamber, and maintain a pool of such liquid in said chamber immersing said closed container, means to detect any pressure differential between said chamber and said container, and a capillary tube venting said chamber above the pool of liquid in said chamber.

4. An apparatus for continuously detecting vapor pressure of liquids comprising a measuring chamber, a closed container suitable for containing a standard fluid in said measuring chamber, means to continuouslyv introduce liquid into said chamber, remove such liquid from said chamber and maintain a pool of such liquid in said chamber immersing said closed container, means to d-etect any pressure differential between said chamber and said container, a capillary tube venting said chamber above the pool of liquid in said chamber, and lmeans to provide a constant temperature bath immersing said measuring chamber.

5. An apparatus for continuously detecting vapor p ressure of liquids comprising a measuring chamber, means to Continuously introduce liquid into said chamber, remove such liquid from said chamber, and maintain a pool of such liquid in said chamber, a closed container suitable for containing a standard uid, a differential pressure detector connected to detect any pressure differential between said container and said measuring chamber, and means to provide a constant temperature bath immersing said chamber, surrounding said container, and immersing said differential pressure detector. l

6. An apparatus for detectiing vapor pressure of liquids comprising a measuring chamber, a closed container suitable for containing a standard ffuid within said measuring chamber, means to continuously introduce liquid into said chamber, remove such liquid 'from said chamber, and maintain `a pool of such liquid in said chamber immersing said closed container, a differential pressure detector connected to detect any pressure differential between said chamber and said container, and a constant temperature bath immersing said measuring7 chambr and said differential pressure detector.

7. A method of detecting any variation in the vapor pressure of a stream of liquid from a target value cornprising the steps of placing a standard liquid having substantially the same composition as said stream and having a vapor pressure at said target value in a closed container, forming a pool with said stream of liquid, immersing said closed container with the said standard 'liquid therein in said pool, continuously introducing liquid from said stream into said pool, continuously removing liquid from said pool, and detecting any vapor pressure diierential between the liquid in said pool and in said closed container.

8. A method of detecting any variation in the vapor pressure of a stream of liquid from a target value at a predetermined temperature comprising the steps of placing a standard liquid having substantially the same composition as said stream `and having a vapor pressure at said target va'lue in a closed container, forming a pool of liquid with said stream of liquid, maintaining the teinperature of said pool at said predetermined temperature,

immersing said closed container with the standard liquid therein in said pool, continuously introducing liquid from said stream into said pool, continuously removing liquid from said pool, and detecting any vapor pressure differential `between the liquid in said pool and in said closed container.

References Cited by the Examiner DAVID SCHONBERG, Primary Examiner.

RICHARD C. QUEISSER, LOU-IS R. PRINCE,

Examiners. 

1. AN APPARATUS FOR CONTINUOUSLY DETERMINING VAPOR PRESSURE OF LIQUIDS COMPRISING A MEASURING CHAMBER, A CLOSED CONTAINER SUITABLE FOR CONTAINING A STANDARD FLUID WITHIN SAID MEASURING CHAMBER, MEANS TO CONTINUOUSLY INTRODUCE LIQUID INTO SAID CHAMBER, REMOVE SUCH LIQUID FROM SAID CHAMBER, AND MAINTAIN A POOL OF SUCH LIQUID IN SAID CHAMBER IMMERSING SAID CLOSED CONTAINER, MEANS TO DETECT ANY PRESSURE DIFFERENTIAL BETWEEN SAID CONTAINER AND SAID CHAMBER, AND MEANS TO PROVIDE A CONSTANT TEMPERATURE BATH IMMERSING SAID MEASURING CHAMBER. 